The present invention relates to semiconductor devices and methods of assembling semiconductor devices and, more particularly, to methods for improving heat dissipation of semiconductor devices.
Conventional techniques for forming semiconductor devices for high current and/or high-power applications typically involve mounting the power die onto a die pad, which functions as a heat sink to improve dissipation of heat generated by the power die. A die pad, typically formed of metallic material, of a relatively high thickness is required.
Currently, there are two principal techniques to achieve this increased-thickness heat sink, either using a lead frame of increased thickness or using a dual-gauge lead frame. Implementing either technique makes the lead frame significantly more expensive than would otherwise be necessary, thereby making the overall cost of the packaging process undesirably high. The increased thickness also introduces its own limitations into lead frame design and manufacturing as well as for the package layout. For instance, the lead pitch is related to the lead frame thickness requiring a certain etching or stamping aspect ratio.
Use of a dual-gauge lead frame, requires the lead frame to have two or more portions of differing thicknesses. Some techniques require the two lead frame portions be attached to one another, where the portion having the higher thickness functions as a heat sink. This is a relatively complicated arrangement, requiring relatively complex manufacturing processes, also leading to higher cost and presenting some design limitations.
For semiconductor products such as PQFN (Power Quad Flat No Lead) products, the step(s) required for attaching the die in the semiconductor device must be done prior to taping the lead frame in order to avoid damage of the tape in subsequent process steps, such as the curing and/or re-flow processes. Taping the lead frame with the die attached can lead to handling difficulties. For instance, where a die is disposed on a lead frame, the die has to be protected from damage or scratching during the lead frame taping process. Also it may not be possible to design and use a universal tool/jig in the taping machine because each process will likely require a different layout on the lead frame. That means a higher tooling cost and increased difficulty in tooling conversion when products change. Further, the strip warpage of a lead frame with a die is greater than a lead frame without a die, thus leading to higher probability of incidences of lead frame jamming in the taping machine.
Thus, it would be advantageous to develop one or more new manufacturing techniques to lower lead frame cost, particularly in high-power applications, and eliminate the aforementioned handling issues and design limitations. Additionally, it would be advantageous if these one or more new manufacturing techniques could allow more flexibility for mixing and matching of components from techniques normally considered incompatible, or at least difficult to integrate, for semiconductor packaging. It also would be advantageous if these one or more new manufacturing techniques maintained or improved upon currently available performance for heat dissipation.